Passive Wireless Sensor for Measuring AC Electric Field in the Vicinity of High-Voltage Apparatus

A passive wireless sensor for the measurement of ac electric fields emanating from high-voltage (HV) apparatus using radio-frequency (RF) resonant cavities is presented. The sensor is composed of a coaxial transmission line resonator, where the resonant frequency is perturbed by capacitively coupled varactors. HV apparatus induces a bias voltage on the varactors through capacitively coupled electric fields. A printed circuit board on the top of the cavity provides coupling between the cavity and varactors and also between the varactors and the external electric field. The sensor is designed with a resonant frequency in the range of 2.4-2.5 GHz in the industrial, scientific, and medical band. Using capacitive coupling to an HV transmission line with 60 Hz at 1.7 kV, a resonant frequency shift of 7.2 (kHz)/12.5 (V/m) was obtained. The resonance frequency is determined remotely by sending pulses of RF signal to the sensor and recording the ring back of the resonator. RF pulses, with a frequency of 2455 MHz, are used to remotely interrogate the sensor at a repetition rate of 2.5 MHz and a pulsewidth of 100 ns. Downconverting the received signal, a ring back signal is recorded whose analysis determines the resonant frequency.